Author
Topic: A new way to visualize pH and buffer capacity (Read 1925 times)

When I was running a few days ago I thought of an analogy that does a really good job illustrating the workings and behavior of pH buffers and what happens when they are mixed.

Envision a system where there is a reservoir that has a large diameter at a certain height and that diameter then reduces to a narrow tube above and below it. Connected to that reservoir is a sight glass.

Now think of the pH of the system as the water level seen though the sight glass. Just looking at that water level will not tell you how much water the reservoir can hold. This is the same with pH. pH does not tell you how well it is held in place. You’ll have to add a measured amount of water to the system to judge the size of the reservoir by observing the change in water level. A small reservoir will result in a large change of the water level while a large reservoir will cause only a small change in water level.

The same is true for pH and what happens to pH when you add a known amount of an acid or a base to a substance of unknown buffer capacity.It is also true that the buffer capacity of that model with the reservoir depends on the current height of the water level. As seen below the bulge, where the water level will only change a little, may sit at different heights for different systems. Just like a pH buffer has its best buffer capacity at a certain pH range.

Now use this model to represent malt and water:

Malt has a large buffer capacity and in this case its “natural” pH is 5.5 This is the distilled water mash pH. Water , on the other hand, is a weaker buffer than malt. And in this case the added water has a pH of 8.

When you dough-in you connect both systems and the water level (I.e. pH) settles at a point between the grist’s pH and the water’s pH. At what point exactly depends on the size of the respective reservoirs (i.e. the buffer capacities). For the malt that buffer capacity is largely determined by its weight and for the water it is determined by its volume and alkalinity.

I think I’ll have to expand further in these models in the future since they do seem to work better than the model with the springs that I have been using before.

And you could alter/exchange the "reservoir" width of both grain and water to illustrate how using too much water during sparging (or steeping) a smaller amt of grain can cause the pH to rise (e.g., partial grain brewing with resultant tannin extraction, astringency, etc.).

nice. would recommend in next revision to make the sight glass level of the mash in the original diagram at same level as the resevoir (5.5). this same concept can be used for temperature infusions as well since specific heat capacity of a mash (grain and liquid) and water only are different. (though i think it is easiest to follow denny's method of just adding boiling water till you get to your target temperature)

nice. would recommend in next revision to make the sight glass level of the mash in the original diagram at same level as the resevoir (5.5).

Yes, that is a mistake. I have a few more of these diagrams and this was just a copy and paste error.

Quote

this same concept can be used for temperature infusions as well since specific heat capacity of a mash (grain and liquid) and water only are different.

Yes it can. But for those models you don't have these bulgy reservoirs. the bulges are specific to weak acids and bases where you can ran out of "capacity" once the pH is too high or too low. I also think that the concept of thermal mass is already fairly intuitive to brewers but I felt that the subject of pH and buffers is not since it is not something we can easily observe on a daily basis.

richardt, you are correct. When you sparge the reservoir for the mash/grist gets progressively smaller and the water can take over.

what also works well in these models is measuring pH. To do that you need to attach a "sight glass" to the system which itself does change the water level and thus the read pH. This is not a problem in well buffered systems since the sight glass will be very thin, but it is a problem with poorly buffered substances. This is why it is so difficult to get a stable pH reading in RO or distilled water.

I agree thermal mass balance is more easily visualized. I do like the diagram idea as well for visualization. The other alternative is to generate calculations on h+, oh- ions available which is logarithmic to the ph. would work but not really worth the effort. conceptually your diagrams are easy for people to understand.

The illustration as suggested may not be wholly accurate since the mash is lautered before the next infusion...

Yes, the reservoir for the mash pH buffer gets progressively smaller as you drain the mash. It's shape may also change since you change the make-up of acid and bases in the mash. But that would only be a minor detail. In addition to that, its shape and size also changes during the mash as more or less buffers are created. But we should not worry about that.